1. Field of the Invention
The present invention relates to an apparatus (herein referred to as a “gas sorption/desorption analyzer”) adapted to measure the adsorption and absorption (collectively referred to as “sorption”) and desorption properties of a materials capable of gas sorption, and more particularly to an apparatus for measuring the hydrogen adsorbing, absorbing or desorbing properties of hydrogen absorbing alloys, hydrogen adsorbing carbons, and oxygen absorbing substances. The apparatus is unique in the following respects: 1) it can perform these measurement in a precise manner at high gas pressures (0 to 200 atm), 2) it performs these measurement by applying precise aliquots of gas at specific pressures using an automated pressure regulator, 3) it incorporates many unique features which enhance the precision, utility, and safety of the measurement. These features are described below in the summary of the invention.
Note: all pressures in atm are absolute pressures (atma). PID means Proportional Integral Differential.
2. Description of Prior Art
In recent years, much attention has been directed to hydrogen absorbing alloys for the negative electrodes of alkali batteries and for gaseous hydrogen storage. More recently, there has been an increased interest in complex hydrides and carbon materials for gaseous hydrogen storage. Typical examples of hydrogen absorbing alloys are LaNi.sub.5, MmNi.sub.2 Co.sub.3 (wherein Mm is a misch metal). In the case of LaNi.sub.5, when hydrogen is absorbed it can form a solid solution alloy LaNi.sub.5H.sub.x, as well as a metal-hydride LaNi.sub.5H.sub.6, at near ambient temperatures and pressures. An example of a complex hydride that may be used for gaseous hydrogen storage is NaAlH.sub.4 doped with titanium by reaction with TiCl.sub.3. This material can release and re-absorb more than 3 weight percent hydrogen through solid-state processes of decomposing and reforming NaAlH.sub.4. Hydrogen gas can also be absorbed onto and desorbed from graphite and other carbon materials low temperatures (less than −100.degree. C.). Potential applications for these materials are rechargeable alkali batteries, hydrogen storage for use with fuel cells, gas chromatographs, etc. Many of the hydrogen sorption/desorption properties of these materials can be modified and, ultimately, tailored to suit the desired application. For example, the pressure and temperature at which hydrogen sorption and desorption takes place in many hydrogen absorbing alloys can be changed by through minor changes in the alloy composition. Therefore, it is very important to measure the hydrogen absorption/desorption properties of all of these types of materials.
One aspect of the hydrogen sorption (or desorption) properties of a material is the thermodynamics of hydride formation (or decomposition). This is determined by measuring a pressure-composition isotherm (PCT) diagram. FIG. 1A shows an idealized version of such a PCT measurements. This diagram represents the relationship between the pressure (ordinate) and the amount of hydrogen absorption (abscissa) at three different sample temperatures. By performing such measurements at several different temperatures the enthalpy of hydride formation can be determined. This is done by plotting the natural logarithm of the plateau pressure (flat portion of the PCT diagram) versus inverse temperature resulting in a van't Hoff diagram. The enthalpy of hydride formation is taken from the slope of this plot. FIG. 1B shows an idealized version of such a van't Hoff diagram.
Another aspect of the hydrogen sorption (or desorption) properties of a material is the kinetics of hydride formation (or decomposition). This is simply the rate of hydrogen sorption or desorption from the material. This is generally determined by measuring the change in hydrogen pressure versus time in a fixed volume containing the sample. If the volume of the vessel and the volume and mass sample are known the amount of hydrogen sorption or desorption by the sample can be quantified. FIG. 2 shows an example of such a kinetics measurement for La.sub.2Mg.sub.17. By performing such measurements at several different temperatures the activation energy of hydride formation can be determined. This is done by plotting the natural logarithm of the rate (usually the initial linear portion of the kinetics measurement) versus inverse temperature resulting in an Arrhenius diagram. The activation energy of hydride formation is taken from the slope of this plot. FIG. 3 shows an example of such a van't Hoff diagram for La.sub.2Mg.sub.17.
A third and very important aspect of the hydrogen sorption (or desorption) properties of a material is the cycle life. That is, how the hydrogen capacity and kinetics hold up with repetitive hydrogen sorption and desorption cycles. In practice, this consist of making a series of kinetics measurements and quantifying the changes in capacity and kinetics as a function of the number of cycles.
The most common way to measure these hydrogen sorption properties of a material is to measure the drop in pressure in a calibrated volume as hydrogen is adsorbed or absorbed by a test sample of the material. Likewise, desorption properties are determined by measuring the increase in pressure in a calibrated volume as hydrogen is desorbed from the test sample into the volume. The quantity of hydrogen absorbed (adsorbed) or desorbed in each measurement is found from the equation of state of gaseous hydrogen. The equation of state is well approximated by the ideal gas law at pressures below about 10 atm. Above this pressure, non-ideal gas laws or tables of experimentally determined values may be used. In any case, it is necessary to know three parameters to determine the quantity of hydrogen absorbed (adsorbed) or desorbed. These are, the pressure, temperature and volume of the gas. By holding the volume and temperature constant, the quantity of hydrogen is determined simply by measuring the pressure. Knowing the mass of the sample it is then possible to determine the mass concentration of hydrogen that has been absorbed (adsorbed) or desorbed by the sample. If the composition of the sample is well known, then the stoichiometry of hydrogen in the sample may also be determined from the measured concentration.
In a PCT measurement, the sample is dosed with small “aliquots” of hydrogen from a small volume or desorbed into a small volume such that only a small fraction of hydrogen is absorbed (adsorbed) or desorbed at one time. A sorption PCT diagram is measured by increasing the pressure in each aliquot of hydrogen applied to the sample in a step-wise fashion. Similarly, a desorption PCT diagram is measured by decreasing the pressure in a step-wise fashion, in the small volume into which the sample is desorbed. The conventional apparatus for performing such measurements is referred to as a Sieverts' device. Such a device is shown in the schematic illustration of FIG. 4.
While this simple device has been used extensively over the years to make hydrogen sorption and desorption measurements, there are a number of technical issues which plague the accuracy and ease with which these measurements can be made. The following is a list and description of the most important problems effecting the prior art. The present invention overcomes each of these problems through the use of unique, effective, and simple, hardware or software solutions.
Problem 1: Automation. Most measurements require data collection over long periods of time and may involve large numbers of repetitive operations (such as delivering aliquots in a PCT measurement, or switching between sorption and desorption in cycle-life measurements). This stresses the importance of using automation in such measurements. The present invention comprises an automated gas sorption/desorption analyzer that employs computer controlled operations and data collection.
Problem 2: Uniform PCT Gas Aliquots. Providing an evenly spaced distribution of sorption/desorption measurement points along PCT curve is of critical importance because without a detailed and even distribution of measurements the low pressure region of the PCT plot will not be well resolved. The result is that the solid solution portion of the hydrogen behavior may not be observed at all. In the worst case, which has been known to occur, changes in the equilibrium plateau pressure identified with hydride phase transitions could be missed entirely.
The classic Sieverts apparatus uses a fixed applied pressure equal to the highest pressure of the measurement for sorption and usually vacuum for desorption. This creates the undesirable situation of requiring very large aliquots of gas for the initial portion of sorption measurements and the reverse for desorption. The best approach to resolve this problem is to be able to vary the applied pressure at will. One method which addresses this problem is described in U.S. Pat. No. 5,591,897. In that method three automated valves work in conjunction to step the applied pressure up or down to the desired aliquot pressure. The method consists of alternately filling (or emptying) a first volume with hydrogen by opening a primary valve, closing the primary valve and then opening a secondary valve to the reservoir volume. In this manner the pressure is stepped up or down to the desired pressure prior to application of the aliquot to the sample. The disadvantage of this method is that every point measured on a PCT curve requires a tedious process of multiple valve operations in order to reach the desired pressure. This process takes time, increases the noise level of the measured response produced by such devices, and greatly increases wear on the automated valves. In addition, during the time that it takes for this process to be completed hydrogen interaction with the material of interest may continue to proceed within the sample volume. This will contributes to a certain amount of error in the measurements of hydrogen capacity.
Another method uses a needle valve to increase or decrease the supply pressure. The problem with this method is that the supply pressure increases or decreases without feed-back control. Because of this, the supply pressure will not be adjusted with respect to changing conditions during gas sorption or desorption to or from the sample. For example, during a hydrogen absorption PCT measurement of a metal hydride the equilibrium pressure will rise until the plateau pressure is reached. The supply pressure, on the other hand, will continue to rise. The increasing difference between the equilibrium pressure and the supply pressure will cause the data points to spread out as the measurement continues. Important phase change information towards the end of the plateau may be missed. The increasing applied pressure differential may also lead to non-equilibrium conditions as the measurement progresses. In addition, needle valves demonstrate non-linear behavior such that the supply pressure will increase (or decrease) more slowly as the pressure differential across the needle valve decreases. This non-linearity often causes sorption PCT measurements to slow down at high pressures to the point where the measured change in ad/absorbed gas is less than the systematic error. For desorption PCT's with plateau pressures near or below one atmosphere the pressure differential across the needle valve is reduced to the point that there is very little flow and the measurement essentially stalls when the plateau is reached.
These problems are easily overcome in the present invention by using an automated pressure regulator to supply the working gas either at a predetermined pressure or at a specified pressure difference above or below the measured equilibrium pressure. In an alternative embodiment the working gas can be supplied (or removed) at a controlled flow rate using a gas flow controller.
Problem 3: Constant Gas Temperature. Variations in the air temperature in the room in which a volumetric instrument such as that shown in FIG. 4 can produce significant errors in calculating the quantities of hydrogen sorption or desorption from a material. Even if the surrounding air temperature is measured and introduced into the equation of state, the lag time between changes in the temperature of the surrounding air and the temperature of the hydrogen gas in the Sieverts apparatus can be significant enough that the data can not be sufficiently corrected. The problem of variations in the temperature of hydrogen gas in a Sieverts' apparatus can be resolved in two ways. The first is to position a temperature measuring device such as a thermocouple inside of the gas reservoir volume to get an accurate measurement of the gas temperature. This value can then be used in the equation of state to compensate for temporal changes in the temperature of the gas. The problem with this method is that if the apparatus employs several separated volumes and interconnecting tubing the gas temperature in the system may not be equilibrated to the temperature measured in one part of the apparatus. The second solution is to maintain the main body of the apparatus at a fixed temperature. This can be accomplished by submerging the main body of the apparatus in a controlled temperature bath (usually using a water bath). The inherent difficulty with this is that there are commonly many electrical devices such as pressure transducers that are connected to the apparatus which are not compatible with water. Another approach is to have the main body of the apparatus in thermal contact with a temperature controlled thermal ballast. This is typically a large metal plate which is heated and maintained at a fixed temperature slightly above room temperature by an electrical heater and a feedback control system. The difficulty with this concept is to obtain good enough heat transfer throughout the apparatus that the gas temperature is truly constant through out the system.
The best approach and one aspect of the present invention is to regulate the gas temperature by placing the main gas handling portion of the apparatus (gas sorption/desorption analyzer) in an enclosure and regulating the air temperature within the enclosure to a fixed value slightly above room temperature
Problem 4: Gas Temperature When Sample is Heated. To calculate the quantity of gas adsorbed, absorbed (hereafter “ad/absorbed”), or desorbed in a volumetric measurement, it is also necessary to know the temperature of the gas. This is not a problem if the gas temperature is uniform throughout the gas handling system, as provided in the present invention by using a controlled temperature enclosure. However, if the sample is heated, or the temperature is different for the gas handling portion of the apparatus outside of the enclosure, then the exact temperature of the gas is not known. This may cause significant errors in quantifying gas sorption.
The present invention overcomes this problem through two methods. The first is to measure the temperature of the gas within the enclosed part of the gas handling system as well as the temperature of the gas in the sample container. The operator is then given the option to use a weighted average of the gas temperature in calculating the quantity of gas (weighted by relative volume of gas at each temperature). A second, and even more effective manner, to overcome this problem is to reduce the volume of the heated gas to a minimum. This is accomplished in the present invention by using small diameter external gas lines and spacers in the sample container to reduce the volume of gas that at a different temperature than the main body of gas in the enclosed and temperature regulated gas handling system.
Problem 5: Non-ideal Gas Behavior at Elevated Pressures. At pressures above about 20 atm molecular interactions in gases begin to have an effect on the relationship between pressure, temperature, volume of a given quantity of gas. These effects cause a deviation from the “ideal gas” behavior. The properties of the gas are no longer adequately described by the linear Ideal Gas Law. This deviation can cause errors in using pressure measurements of a volumetric device to determining the amount of gas that is ad/absorbed or desorbed by a sample. At pressures above 100 atm, this error may be significant (on the order of 5% or more).
These errors can be successfully overcome by utilizing one of the several non-ideal equations of state developed for gases at high pressure. The present invention includes data analysis software employing automatically calculation of non-ideal gas behavior to correctly determine the hydrogen capacity from changes in pressure.
Problem 6: Small sample quantities. Small samples (<1 gram) and/or samples that ad/absorb only small quantities of gas (50 milliliters) are difficult to investigate using typical volumetric devices that often have calibrated volumes and piping with volumes on the order of 50 milliliters or more. For example, a 0.5 gram sample of a LaNi.sub5.-type alloy subjected to a 2 atm aliquot of hydrogen gas from a 50 milliliter calibrated reservoir volume will be completely hydrided in one step. Under such conditions it would not be possible to measure an equilibrium PCT plateau curve.
The present invention utilizes small gas vessels, spacers, and small internal diameter gas lines (c.a. 1 mm diameter) to reduce the minimum working volumes to about 15 milliliters. This enables the measurement of gas sorption properties of small (<1 gram) samples and samples with limited gas sorption. For larger samples and for desorption, the present invention includes additional calibrated volumes to increase the working volume of the gas sorption/desorption analyzer. These volumes may be accessed by opening valves connecting them to the gas handling system.
Problem 7: Flexibility in Measuring Different Sample Types and Sizes. The calibrated reservoir volume may be too large or too small for the aliquot that is desired. The most obvious example is that a very small volume (15-50 milliliters) is needed for making absorption kinetics measurements using high pressures (100 atm) and a large volume is needed for a desorption kinetics measurement (1 liter at <2 atm). In addition, different types of measurements (PCT, kinetics, and cycle-life) as well as, different types or quantities of samples (1 gram vs. 100 grams) may all have different requirements for the quantity of gas to be supplied or desorbed in an aliquot. Simple Sievert's devices often provide only one or two different calibrated volumes, or required the volumes to be changed manually. The problem with a manual change is that air will be introduced into the system, requiring an additional out-gassing. Also, such hardware changes significantly increase the possibility of system leaks, lost hardware, and possible mistakes or changes in the calibration of the actual volumes being used. There is a great advantage to being able to change to a reservoir of a different volume, even during a measurement.
As mentioned above, the present invention includes at least 3 additional calibrated volumes permanently attached to the gas handling system that are accessed when needed by opening automated valves that connect these volumes to the gas handling system.
Problem 8: Gas Sorption and Desorption Properties May Vary Over a Broad Time Range. It is common that gas sorption or desorption rates will vary by over 2 orders of magnitude during a single measurement. For example, a hydride may absorb hydrogen at 10 wt. %/minute in the first few minutes of a kinetics measurement and continue to absorb hydrogen but at a much lower rate of 0.1 wt. %/minute after several hours. Ideally, one would like to record such data frequently during the rapid part of the measurement and less frequently when changes are occurring slowly. The problem with current data-collection schemes is that data is taken at a fixed time interval. To be able to collect all the pertinent information data must be collected at the shortest required time interval (e.g. 2 seconds). Unfortunately, doing this for a measurement that often lasts hours or days creates enormous data files. And a large portion of the data collected after the most active part of the measurement will be of little value since dozens or hundreds of data point could be equally as well represented by a single data point. Sometimes it is possible to change this interval during a measurement, but this requires operator to be input new values and therefore, they must schedule their time accordingly.
The present invention overcomes this problem by taking advantage of the fact that for typical experiments, gas sorption and desorption rates decrease as a function of time. The present invention includes different algorithms to decrease the frequency with which data is recorded as a function of time. These algorithms are described in a later section.
Problem 9: Constant Pressure Measurements. It is often very useful to be able to make gas sorption or desorption measurement while maintaining a constant active gas pressure over the sample. This is important for kinetic comparison or mechanism studies because kinetics are often strongly influenced by changes in the applied pressure. This is also important in simulating real conditions encountered in applications, such as filling up a hydride bed under a constant hydrogen pressure. Currently, volumetric systems measure gas sorption (of desorption) properties of materials by measuring the pressure drop (or rise) in a calibrated volume which contains the sample material. The need to measure a moderate change in pressure to obtain accurate data means that the sample is subjected to a non-constant pressure during the measurement which may have a significant effect on the results.
The present invention overcomes this problem by employing a computer controlled pressure regulator for gas sorption and a computer controlled back-pressure regulator for desorption measurements. These devices are used to maintain a constant gas pressure over the sample during measurements. The quantity of gas ad/absorbed in a sorption measurement is determined by measuring the drop in pressure in a small (100 milliliter) calibrated volume which supplies the gas to the sample through the pressure regulator. In desorption measurements, the quantity of gas desorbed is determined by measuring the pressure increase in a large (1 liter) calibrated volume to which the gas flows from the sample through the back-pressure regulator.
Problem 10: Changes in Sample Density. Hydrides undergo lattice expansion during hydrogen absorption and lattice contraction during desorption causing minor changes in the calibrated system volumes. These changes are typically not accounted for in the prior art. In addition, it is of interest to be able to measure these expansions and contractions.
The present invention overcomes this problem by performing semi-automatic measurements of the volume (and therefore packing density) of the sample using an inert gas such as He to measure the empty volume of the sample container with and without the sample present. Changes in the sample volume may be made during sorption or desorption experiments by performing a volume measurement with the inert gas at selected intervals during the experiment.
Problem 11: Large Dynamic Pressure Range. The measured pressure often extends over a larger dynamic range than most pressure transducers can measure with good accuracy. In typical devices of the prior art, only one pressure transducer is employed, limiting the range over which experimental measurements can be performed.
The present invention overcomes this problem by utilizing at least two pressure transducers, one covering a low pressure range (e.g. 0-20 atm) and another for higher pressures (e.g. 20-200 atm). Computer controlled automation (PCT, kinetics and cycle-life) is used to determine at any point in an experiment whether the pressure is in the range that should be measured using either a low range transducer or a high range transducer. In the case of the present preferred embodiment of the invention two transducers are used. The pressure is first measured with the high-range transducer. If the high-range transducer indicates a low enough value (below a selected set point), an automated valve is opened to allow the pressure to be measured with the low-range transducer. The automated valve is used to protect the low-range pressure sensor from damage by over-pressurization. In this case the change in the calibration volume is adjusted to account for the additional volume of the low-range transducer, valve and connections.
Problem 12: Elevated Pressure and Temperature Operation. Many materials require temperatures and pressures substantially above ambient conditions (ca. 400.degree. C. and 200 atm.) The prior art has focused on measurements of hydrogen absorption in classic interstitial metal hydrides for hydrogen gas storage and battery applications. In most cases, these materials have equilibrium hydride formation plateaus that are measured either near ambient conditions or at elevated temperatures (e.g. MgH.sub.2 at 300.degree. C.) and pressures of 30 atm or less. It is desirable (particularly in light of new reversible hydride development e.g. Ti doped NaAlH.sub.4) to be able to make measurements at higher pressures and temperatures. The currently available systems either do not go to pressures above about 30 atm, are not accurate at temperatures above 300.degree. C., or are not automated.
The present invention overcomes this problem by using high-pressure components, lines, fittings, and a sample container which are rated for hydrogen service up to 200 atm. The present invention also includes a sample container which is rated for service at up to 400.degree. C. at 200 atm of hydrogen. Errors caused by variations in the gas temperature when operating at this high sample temperature are minimized by reducing the empty volume of the external gas handling system and sample container, as well as offering the option to use a weighted average temperature for concentration calculations. The present invention also includes automated pneumatic valves, and the use of valves in series to minimize the leak rates across the valves when performing high-pressure measurements. Automated valves also have a great advantage with respect to durability and life-time of an apparatus. This is because they are operated in a repeatable and consistent manner which significantly reduces damage to the internal components. Manual valve are subjected to the inconsistent behavior of the operator. In particular, when used with high-pressure the experimenter has a tendency to over-tighten such valve, which will damage the valve seat and cause the valve to leak. Manual systems have been known to operate for only a few experiments before the valves leak to the point that the data is seriously compromised. Automated pneumatic-valves, on the other hand, can be set to operate over thousands of cycles using an air pressure that is appropriate and constant.
Problem 13: Air Exposure. Typical prior-art apparatus have been designed such that the sample cannot be place in the sample container or that the sample container cannot be connected to the apparatus without exposing the sample to air. This is a very big problem for samples that become inactive because of the formation of surface oxides or other coatings and is a serious problem for samples which are highly reactive with air such as Na—Al—H compounds.
The present invention overcomes this problem by using a sample container that is small enough to fit through an entry chamber of a glove box so that it can be loaded with samples in an inert atmosphere such as argon gas. In the present invention, the sample container is comprised of the body of the container with two open ends. This has two advantages, the first is that samples can be easily loaded and more importantly easily removed from the sample container. The other advantage is that different types of end-pieces can be attached to each end. In the preferred embodiment one end-piece has a valve between the body of the sample container and a connector for attaching the sample container to the external gas-handling system of the gas sorption/desorption analyzer. This allows the sample to be sealed under an inert atmosphere while the sample container is removed from the glove box attached to the gas sorption/desorption analyzer and all lines of the gas sorption/desorption analyzer are pumped free of air. In this manner the sample is loaded, transferred and ready for measurements without ever being exposed to air. In the preferred embodiment, the other end of the sample container is sealed with a fitting that contains a tube closed at one end and sealed to the fitting. This tube is known as a thermocouple “well”. It runs up inside of the sample container to make precise measurements of the actual sample temperature. In an alternative embodiment, this end-piece could be replace with another valve and connector end-piece. This allows other devices to be attached in an air-less manner to the sample valve. Such devices could be a turbo-molecular pump, gas or liquid vessel containing a reactive or calibration gas or liquid, a residual gas analyzer, etc.
Problem 14: Safety. Measurements made using high-pressure hydrogen at high temperature and in many cases with highly reactive materials presents many safety issues and challenges that are not properly addressed in much of the prior art.
The present invention comprises several innovative hardware and control logic mechanisms to improve safety. Examples include failsafe mechanisms, such as a pop-top, which will minimize that would otherwise be caused by a build-up of hydrogen and ignition within the gas sorption/desorption analyzer enclosure. Other examples include logical mechanisms, such as temperature limits for the enclosure heating system and sample-container heater which are built into the control software. These mechanisms will be discussed in detail below.